Glutamate is the most prevalent excitatory neurotransmitter in the central nervous system (CNS) of mammals. Glutamate plays an important role in numerous physiological functions such as cardiovascular regulation, perception and recognition, and synaptogenesis as well as learning and memory. As such, in the occurrence of imbalance in glutamate neurotransmission, various nervous and mental diseases such as schizophrenia may be caused, and thus glutamate plays an important role in physiology.
Glutamate mediates synaptic neurotransmission via ionotropic glutamate receptors (iGluR)—i.e., the activation of NMDA receptors, AMPA receptors and kainate receptors involved in rapid excitatory transmission (Nakanishi S. et al., Brain Res. Rev., (1998) 26: 230-235). In addition, glutamate plays a role in subtly regulating excitatory synaptic neurotransmission via the activation of metabotropic glutamate receptor (mGluR).
Metabotropic glutamate receptors (mGluR) consist of eight (8) subtypes and are sub-divided into three groups (Groups I, II and III) according to arrangement, homology and pharmacological property. mGluR5 belongs to Group I, and it is known that mGluR5 interacts with NMDA receptors via various proteins and neurotransmission pathways. Therefore, because the balance of deficiency or hyperactivity of physiological function by NMDA receptors can be regulated via the modulation of mGluR5, to modulate mGluR5 is very important.
Since a variety of pathophysiological processes and disease states affecting the CNS are thought to be related to abnormal glutamate neurotransmission and NMDA receptor malfunction, modulators of mGluR5 receptors could be therapeutically beneficial in the treatment of various CNS diseases. Moreover, because mGluR5 receptor modulators which act through allosteric binding site have some advantages such as subtype selectivity, brain penetration and safety potential, many studies have reported that the mGluR5 positive allosteric modulators were useful for the treatment of schizophrenia and CNS diseases.
International Publication Nos. WO 2008/151184 and WO 2011/035324 disclose benzamide and O-benzyl nicotinamide derivatives as an mGluR5 positive allosteric modulator, respectively. International Publication No. WO 2010/124055 discloses 2-alkyl piperidine derivatives as an mGluR5 positive allosteric modulator, and International Publication No. WO 2011/082010 discloses tetrahydrotriazolopyridine derivative compounds. International Publication Nos. WO 2012/078817 and WO 2012/083224 disclose bicyclic pyrazole and bicyclic triazole compounds as an mGluR5 positive allosteric modulator, respectively.